Optoelectronic devices, such as optical transponders and optical transceivers, for example, generally involve interconnection between an optical component and an electrical component. For example, optical sub-assemblies, such as transmitting optical sub-assemblies (TOSA) and receiving optical sub-assemblies (ROSA), were connected directly to an optical edge of a circuit substrate, such as a printed circuit board (PCB), by directly soldering or epoxying the electrical leads of the optical sub-assembly to signal traces embedded on or in the circuit substrate. Placement of the optical component was thereby limited, resulting in limitations for the design and manufacture of the optoelectronic device. The optoelectronic device also included several electrical components, such as a serializer/deserializer, a clock and data recovery unit, or other high speed electrical components. Generally, these electrical components were mounted on the circuit substrate towards an electrical edge, or card edge connector, of the circuit substrate which was opposite the optical edge. The traces within the circuit substrate were used as the interconnect between the electrical component and the optical component and ran from the optical edge to the electrical component near the electrical edge.
In some cases, the optical component was provided with a short flexible interconnect, such as a flexible circuit. One end of the flexible interconnect was electrically connected to the optical component via the electrical leads, and another end of the flexible interconnect was electrically connected to embedded signal traces at the optical edge of the circuit substrate. As above, the traces ran along the circuit substrate from the optical edge to the electrical component positioned towards the electrical edge. The flexible interconnect generally included two unshielded layers: a trace layer and a ground plane layer. The ground plane layer was generally positioned on one side of the flexible interconnect and a controlled impedance stripline was positioned on the opposite side of the flexible interconnect. Generally, the flexible interconnect was approximately 10 mm in length. The short length of the flexible interconnect limited flexibility and prohibited mounting additional components on the interconnect rather than on the circuit substrate. In turn, the design options for the optoelectronic device, for example, placement of optical components, circuit substrate size and PCB design, were limited.
One or more signal traces ran from the optical edge to the electrical component near the electrical edge regardless of how the optical component was connected to the optical edge of the circuit substrate. The optical component was connected directly or via a short flexible interconnect. The trace on the circuit substrate was generally a line or “wire” of conductive material such as copper, silver or gold, and resided on the surface of or within the circuit substrate. The traces carried electronic signals between the optical component and the electrical component. However, the circuit substrate was generally lossy and the traces resulted in interference among the electronic signals. For example, a transmitting differential signal to the optical component sometimes interfered with a receiving differential signal from the optical component. The electrical signals further experienced interference from other components on the circuit substrate. Although an amplifier was sometimes used when receiving differential signals from the optical component, transmitting differential signals often had low signal strength, and were more susceptible to the interference and the lossy nature of the circuit substrate described above.
Electrical components in addition to those described above were also mounted on the circuit substrate. Some of these additional components were proprietary to the particular optical component or components. As such, the design and manufacture of the printed circuit board would vary depending on the optical component.